1. Field of the Invention
The present invention relates to a method and apparatus for machining a peripheral portion of a printed circuit board so as to work out the contour of the printed circuit board by effecting a relative feed movement between a rotating tool and the printed circuit board, and more particularly, to a contour machining method and apparatus intended to prolong the life of the tool while eliminating any machining failure.
2. Description of the Prior Art
Hitherto, the life of a machining tool (hereafter, referred to as tool) for contour machining of printed circuit boards has been detected by comparing the total machining distance of the tool in use with a predetermined value which has been set in accordance with experience. When the total machining distance of the tool has exceeded this predetermined value, the tool life is deemed to have expired and renewal of the tool is carried out.
The life of the tool, however, varies according to various factors such as the machining speed (feed speed), tool rotation speed and so forth, so that the total machining distance alone cannot provide a satisfactory base for the determination of the life of the tool. Therefore, the above-mentioned set value of the total machining distance, which is deemed to be the tool life, is often too large or small. When the set value is too large, there is a risk that the tool is broken during the machining, resulting in a defective product. On the other hand, when the set value is too small, the tool is used wastefully although the risk of breakage of the tool during machining is eliminated.
Under these circumstances, several methods have been proposed in which electrical current value of a motor for driving a spindle to which the tool is attached is detected and the machining conditions are changed in accordance with the detected electrical current value thereby to prolong the life of the tool. In these known methods, however, no specific consideration is given to the amount of compensation of the tool diameter peculiar to machining of the printed circuit board. In consequence, there is a problem in that the machining precision is undesirably lowered.